Electronic components have been developed that convert analog to digital signals and vice versa. These converters are used in applications that receive inputs from analog sensors and/or in cases where digital signals are used to interface with analog components. An analog to digital converter (hereinafter referred to “ADC”), converts an analog voltage to a digital number. A digital to analog converter (hereinafter referred to “DAC”), converts a digital number to an analog equivalent signal.
In certain applications the speed of the signal conversions is controlling. In some of these cases, the converter must be able to keep up with rapidly changing data. In other applications the ability to resolve between two close analog voltages is controlling. The number of bits that make up the digital number as well as the reference voltage used to implement the ADC/DAC determine the resolution of the ADC/DAC. For example, an eight-bit ADC/DAC ranges from 0 to 255, i.e., 256 values. If the highest number, i.e., 255, is scaled to 5 V, i.e., the reference voltage, the resolution of the ADC/DAC is 19.58 mV. That is, the ADC/DAC can only resolve to within 19.58 mV. If, for the same reference voltage, a ten-bit result is generated the resolution is thereby improved to 4.88 mV.
In some other applications the footprint of the converter is the most important factor. In these cases, the objective is to make the size of the ADC or DAC as small as possible within the geometrical constraints of the semiconductor technology being used. For example, the number of capacitors used in an ADC/DAC may result in a large footprint for the ADC/DAC. FIG. 1 shows an example of the front end of an N-bit DAC found in the prior art. Capacitors are switched in and out to adjust the output of the DAC (daco). The number of capacitors in this type of implementation is N+1, where N is the number of bits. Furthermore, these capacitors must be binary weighted and are usually implemented with a unit capacitor. Thus, the actual implementation requires 1024 capacitors. These capacitors must match to avoid conversion inaccuracies. For example, each successive bit or resolution requires the matching to improve by a factor of 2×. Since matching capacitors in semiconductor technology is proportional to the square root of the area of the capacitors, every additional bit of resolution requires an increase in the capacitor area of 4×.
Serial charge redistribution digital to analog converters were developed to address some of these shortcomings. FIG. 2 shows an example of a prior art D/A converter including two capacitors and three switches that has a smaller footprint than the converter of FIG. 1. However, serial D/A converters, such as the one shown in FIG. 2 are excessively slow for certain applications, and suffer from non-ideal behavior due to charge injection from the switches.
What is needed is an ADC/DAC converter that provides sufficiently fast conversions, with sufficiently fine resolution, a small footprint, and reduces the non-ideal behavior of switches.